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Two researchers argue that climate is the primary driver of aggression

They say there's a "general trend" between proximity to the equator and violence

One of the uncomfortable facts of life is that some countries — and within the U.S., some states — are more violent than others. Figuring out what makes societies and communities more violent is a big-time quest for social scientists, since if you know what leads to killing, you'll have a better shot at defusing it. Drawing on a host of studies, a new paper (PDF) in Behavioral and Brain Sciences -- lead-authored by Paul A. M. Van Lange of Vrije Universiteit, Amsterdam -- proposes a model for understanding violence with the only somewhat forced-sounding acronym CLASH, short for CLimate, Aggression, and Self-control in Humans.

The authors argue that climate -- in the form of the culture it gives rise to -- is the primary driver of aggression. Basically, the authors say that living in a place with greater seasonality and lower temperatures prompts people to be more future-oriented and more greatly value self-control, which the researchers call the "slow life" strategy, as opposed to the "fast life" strategy associated with hotter, less seasonal places, many of which are clustered around the equator. "Climate is one of the broad and ubiquitous variables that is likely to be a powerful cause of culture (how groups organize, the norms they share, and how individuals think and behave)," Van Lange wrote to Science of Us in an email.

Climate shapes planning lots of ways, he says, like in how you hoard for the winter or plan for the next season as individuals, families, and communities. And while winter kills off lots of environmental threats, hotter places have more venomous animals and infectious diseases, adding to their unpredictability. "We do acknowledge that various other factors are linked to climate -- such as prosperity -- and they are also partially caused by climate," Van Lange said.

According to CLASH, violence comes from people assuming that they're going to have shorter lifespans, less planning for the future, and less self-control. "We see evidence of a faster life strategy in hotter climates with less temperature variation -- they are less strict about time, they have less use of birth control, they have children earlier and more often," study co-author Brad Bushman said in a statement.

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The data around violence and climate is serious. A 2013 meta-analysis analyzed 60 studies on the relationship between climate and human conflict, and the authors found that for each standard deviation, the median effect was a 14 percent increase in conflict between groups and a percent increase for individuals.

And as Van Lange and his colleagues argue, there's a "general trend" between proximity to the equator and violence. They cite a 2013 UN global homicide report (PDF) that finds startling regional differences: There are 26 homicides per 100,000 people in Central America and 18 per 100,000 in Middle Africa, compared with 5 per 100,000 in Europe and Northern America. You find it at the regional level as well: southern Europe (Turkey, Albania, Montenegro) has more murders than Scandinavia, and in the U.S., the South has more crime than the North (with the notable exception of Alaska, where there are outlier levels of rapes and aggravated assaults). Indeed, the authors report, there are even more mafia-related killings in Southern than Northern Italy. (But, you have to note, Africa and Central America are way poorer than North America and Europe, and the inequality -- another strong correlate (PDF) to violence -- within those countries is astounding.)

This isn't the first model to attempt to explain the link between temperature and violence. The General Aggression Model (PDF) argues that warmer temperatures put people into more highly aroused states, which primes impulsive, potentially aggressive behavior. Routine Activity Theory contends that crime goes up with temperature because people are more likely to be out in public spaces, making them more exposed to potential conflict. Van Lange and his colleagues argue that these models are incomplete: the General Aggression Model suffers from evidence (PDF) indicating that heat-induced hostility isn't really that common (according to CLASH, the climate creates the culture, and the culture creates the predisposition to violence -- it's not people reacting directly to the heat, in other words). The Routine Activity Theory founders, they argue, since so much violence happens between friends and family inside the home, and because people getting together in public places actually increases (PDF) social cohesion, among other factors. (But, zooming out a little, it does seem that CLASH and Routine Activity Theory are compatible with one another -- how could people spending more time outside decrease the odds of public violence?)

There are lots of limitations to Van Lange and his colleague's paper. For one, it's presenting a model in the form of argument, rather than presenting original empirical research (with the logic that such a model will help frame further research). Beyond that, the most obvious thing is there's something quixotic in trying to reduce the complexity of human-on-human violence to a single variable. It's kind of like a radical leftist saying that the violence so present around equatorial states is simply the legacy of imperialism and colonialism, or a neoconservative saying that if we could only get all of these countries to be democracies, then we'd arrive at democratic peace (PDF). There are so many inputs into violent acts (and violent cultures like the "honor culture" (PDF) ethnographers have described in the American South) that attributing violence mostly to a single variable seems like a reach.

Also, there's ample evidence that contradicts the "temperature shapes culture which shapes violence" argument that animates CLASH. Just look at seasonal American cities: murders spike in Chicago and New York in the summer, and a Governing analysis of 384 law-enforcement agencies found that major offenses went up by 10 percent between June and August compared to the rest of the year. It's not like every June Chicagoans suddenly transform to "low-trust," "fast life" strategies and then go back to "slow life" long-term planning when the temperatures plunge in November. Something more short-term is taking place.

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Finally, there's the matter of saying that people's behaviors are a product of their regions, which is awful close to saying that behavior is a product of their ethnic culture. When I asked Van Lange how CLASH avoids that determinism, he replied that he and his co-authors "think it is ultimately climate that is an important cause of culture, and therefore social and individual thinking and behavior," noting research that finds that people who move from warm climates to colder climates "adapt quickly" to their new cultural environments. "For example, research has shown that people who immigrate from a low-trust nation such as Turkey to a high-trust nation in Northern Europe will rapidly show greater trust in people in general," he argues. "People are in that sense 'very adaptive' and we think that what research has shown for trust will be also be largely true for long-term orientation and self-control." But while research does indeed find that immigrants (and especially their kids) adapt to the social-trust norms of their adopted country, there's a jump in the logic to say that it's the climate that's doing the work. People adapt to cultures, but that's not proof that climate shapes violence.